1. Field of the Invention
The present invention relates to a sheet braking mechanism for a sheet punching and embossing machine.
2. Description of the Related Art
Sheet processing machines, such as sheet punching machines, for example, typically include a feeder, processing stations, and a delivery unit. A sheet lying on top of a stack of sheets in the feeder is fed to a transport system. The transport system transports the sheet through the processing station to the delivery unit. A known transport system includes rotating gripper carts. The gripper carts include a transverse rod on which grippers are arranged. The grippers grab a forward edge of the sheet. The gripper carts include ends that are fastened to a side chain control which guides the gripper carts through the machine. For processing and delivery, the sheet must be braked to a standstill. This is performed by braking the gripper carts, and by additional sheet brakes. The sheet processing machines may be sheet printing machines or sheet punching and embossing machines, for example.
Punching refers to a cutting process using self-enclosed geometrical blanking shapes, which may be circular, oval, or polygonal, as well as any kind of special shape. This field also includes the practices that are performed in the further processing of printed matter, such as punching with perforation dies, rounding of edges, and register punching. The punching is performed against a punching backing or against a male die. In addition, shearing steps may also be performed. Packaging materials, such as paper, a carton, cardboard or corrugated cardboard, are primarily punched out in sheet form. However, grooved lines or blind embossings can also be made in the copies during the punching process. This complex process required the sheets to be individually punched. Since the end products are packaging items requiring highly technical and graphical design (such as packages for cosmetics, cigarettes, pharmaceuticals, foods, etc.), special requirements are placed on not only the packaging materials themselves, but also on the punching dies. For example, extremely precise and reliable punching machines having very small tolerances are required for optimal results. These requirements are best met using a flat-bed punching machine in which printed sheets that are stacked on a pallet are fed to the punching machine. The flat-bed punching machine includes an orienting mechanism in which the sheets to be punched are oriented true to size and picked up by a gripper cart and precisely positioned in the punching mechanism between a firmly mounted bottom table and a vertically movable upper table.
In known sheet punching and embossing machines which are used for punching, waste stripping, embossing and stacking of sheets of paper, cardboard, and other materials, it is known to transport the sheets through the individual stations of the machine using gripper carts. A gripper cart includes a gripper bridge on which the grippers are fastened, and the gripper cart grabs the sheets at a front end. In addition, a gripper cart includes lateral driving carts, which are connected to endless chains of the transport system and move the gripper cart through the machine. With this type of movement of the sheets through the machine, continuous operation is achieved in the individual consecutively arranged stations of the machine, such as a punching station, a waste stripping station, and a copy separation station.
Such a flat-bed punch is disclosed in DE 30 44 083 A1. Two tables are provided with cutting and grooving tools or the corresponding counter tools, by which the final copies are punched from the sheets, guided in a timed manner between the table surface, and at the same time the grooves required for clean folding are pressed into the final copies. At the following waste stripping mechanism, the waste is removed mechanically by stripping tools. Finally, depending on the configuration of the machine, the punched copies are separated in a copy separation mechanism.
In the punching station, the waste stripping station, and the copy separation station, the sheet must be braked from the transport speed to a standstill. Since the sheet that is already weakened by the preceding cutting process arrives at the stations at a high speed, merely slowing down the front gripper rod may cause its rear portion to buckle upward. This is prevented by additional braking devices which act on the surface of the sheet. A taut sheet can be processed with greater precision and transported more easily.
Two different types of sheet braking device are known in the related art. DE 695 00 514 T2, for example, shows a sheet braking device that includes brushes. The braking brushes are slanted in the direction of sheet transport and exert a braking force on the sheet by slight pressure.
In an alternative solution to this mechanical sheet brake, pneumatic sheet brakes are used, e.g., as disclosed in EP 1 431 011 B1. The pneumatic sheet brakes are arranged in close proximity to the sheet and provide a partial vacuum, which acts as a braking force on the sheet moving past the pneumatic sheet brake. Pneumatic sheet brakes have an advantage over mechanical ones in that the sheet surface is not impaired marks on the sheet surface can be avoided marks. Another known pneumatic sheet brake is disclosed in DE 10 2005 016 783 A1.
DE OS 27 20 674 discloses a rotary printing machine having a blowing device. The blowing device functions like a sheet brake and includes a strip of blowing nozzles, which can be moved in the same direction or in a direction opposite to the sheet transport direction. This is performed to adjust the sheet brake for the particular sheet delivery speed. The nozzles of the strip are supplied with compressed air and produce a partial vacuum on the underside of the sheet due to the aerodynamic paradox, so as to impart a braking force to the sheet.
DE 199 26 401 C1 discloses a method of controlling an axially adjustable sheet braking mechanism. Here, the sheet brakes can be moved in the direction of or opposite to the sheet transport direction, depending on the lengthwise format of the sheet, using guide carriages and lengthwise drive units, such as a screw.
DE 198 35 529 A1 discloses a sheet braking device for corrugated cardboard machines. The device includes suction strips which rotate in a circular orbit, and a guide roller arranged opposite to the suction strips. The suction strips are arranged below the plane of sheet transport and act on the sheet from beneath, while the guide roller is arranged above the sheet transport plane and acts on the sheet from above. The roller is used to guide the sheet.
The drawbacks of the sheet braking mechanisms of the related art include their limited braking capacity, the complicated positioning process of the braking elements after a tool change, and their complicated adjustment process to adapt to the different sheet formats. Another drawback is that there is a danger of collision between the sheet braking mechanisms and the tools when changing the print run and the tooling.